contractcourt: add rapid derived fuzz test for HtlcAuxBlob

In this commit, we add a rapid derived fuzz test for the HtlcAuxBlob
test. This uses the rapid (randomized property testing) into Go's built
in fuzzer. This wrapper will use the fuzz stream, and pass that into
rapid where the stream is used to make structured test inputs which are
tested against the existing properties.

This can be done more widely in the codebase, we pick a simple example
to port first before tackling others.

contractcourt/taproot_briefcase_test.go

@@ -127,12 +127,9 @@ func TestTaprootBriefcase(t *testing.T) {
 	require.Equal(t, testCase, &decodedCase)
 }
 
-// TestHtlcAuxBlobEncodeDecode tests the encode/decode methods of the HTLC aux
-// blobs.
-func TestHtlcAuxBlobEncodeDecode(t *testing.T) {
-	t.Parallel()
-
-	rapid.Check(t, func(t *rapid.T) {
+// testHtlcAuxBlobProperties is a rapid property that verifies the encoding and
+// decoding of the HTLC aux blobs.
+func testHtlcAuxBlobProperties(t *rapid.T) {
 	htlcBlobs := rapid.Make[htlcAuxBlobs]().Draw(t, "htlcAuxBlobs")
 
 	var b bytes.Buffer
@@ -142,5 +139,18 @@ func TestHtlcAuxBlobEncodeDecode(t *testing.T) {
 	require.NoError(t, decodedBlobs.Decode(&b))
 
 	require.Equal(t, htlcBlobs, decodedBlobs)
-	})
+}
+
+// TestHtlcAuxBlobEncodeDecode tests the encode/decode methods of the HTLC aux
+// blobs.
+func TestHtlcAuxBlobEncodeDecode(t *testing.T) {
+	t.Parallel()
+
+	rapid.Check(t, testHtlcAuxBlobProperties)
+}
+
+// FuzzHtlcAuxBlobEncodeDecodeFuzz tests the encode/decode methods of the HTLC
+// aux blobs using the rapid derived fuzzer.
+func FuzzHtlcAuxBlobEncodeDecode(f *testing.F) {
+	f.Fuzz(rapid.MakeFuzz(testHtlcAuxBlobProperties))
 }